We can pumped gas into the balloon such as helium, hydrogen, oxygen, nitrous oxide (laughing gas) , and air
but helium is safer because it nonflammable.
Although helium gas was expensive than hydrogen and other , it worth for asset and security of life.

A balloon sky that filled with gas is lighter than air, that can make it rise up in the air.

When I was young I really like to look up to the sky and try to found balloons,
when it float and combine it is very beautiful.